The isolation of coated vesicles from protoplasts of soybean

The Beginning of the End: Initial Steps in the Degradation of Plasma Membrane Proteins

The plasma membrane (PM), as border between the inside and the outside of a cell, is densely packed with proteins involved in the sensing and transmission of internal and external stimuli, as well as transport processes and is therefore vital for plant development as well as quick and accurate responses to the environment. It is consequently not surprising that several regulatory pathways participate in the tight regulation of the spatiotemporal control of PM proteins. Ubiquitination of PM proteins plays a key role in directing their entry into the endo-lysosomal system, serving as a signal for triggering endocytosis and further sorting for degradation. Nevertheless, a uniting picture of the different roles of the respective types of ubiquitination in the consecutive steps of down-regulation of membrane proteins is still missing. The trans-Golgi network (TGN), which acts as an early endosome (EE) in plants receives the endocytosed cargo, and here the decision is made to either recycled back to the PM or further delivered to the vacuole for degradation. A multi-complex machinery, the endosomal sorting complex required for transport (ESCRT), concentrates ubiquitinated proteins and ushers them into the intraluminal vesicles of multi-vesicular bodies (MVBs). Several ESCRTs have ubiquitin binding subunits, which anchor and guide the cargos through the endocytic degradation route. Basic enzymes and the mode of action in the early degradation steps of PM proteins are conserved in eukaryotes, yet many plant unique components exist, which are often essential in this pathway. Thus, deciphering the initial steps in the degradation of ubiquitinated PM proteins, which is the major focus of this review, will greatly contribute to the larger question of how plants mange to fine-tune their responses to their environment.

Thermophoretic migration of vesicles depends on mean temperature and head group chemistry

A number of colloidal systems, including polymers, proteins, micelles and hard spheres, have been studied in thermal gradients to observe and characterize their driven motion. Here we show experimentally the thermophoretic behaviour of unilamellar lipid vesicles, finding that mobility depends on the mean local temperature of the suspension and on the structure of the exposed polar lipid head groups. By tuning the temperature, vesicles can be directed towards hot or cold, forming a highly concentrated region. Binary mixtures of vesicles composed of different lipids can be segregated using thermophoresis, according to their head group. Our results demonstrate that thermophoresis enables robust and chemically specific directed motion of liposomes, which can be exploited in driven processes.

Thermal gradients are shown to provide a robust and chemically specific driving force to liposomes. Here the authors show controlled direction of migration of unilamellar lipid vesicles by varying the temperature in the suspension and the exposed polar lipid head groups.

Conserved and plant-unique mechanisms regulating plant post-Golgi traffic

Membrane traffic plays crucial roles in diverse aspects of cellular and organelle functions in eukaryotic cells. Molecular machineries regulating each step of membrane traffic including the formation, tethering, and fusion of membrane carriers are largely conserved among various organisms, which suggests that the framework of membrane traffic is commonly shared among eukaryotic lineages. However, in addition to the common components, each organism has also acquired lineage-specific regulatory molecules that may be associated with the lineage-specific diversification of membrane trafficking events. In plants, comparative genomic analyses also indicate that some key machineries of membrane traffic are significantly and specifically diversified. In this review, we summarize recent progress regarding plant-unique regulatory mechanisms for membrane traffic, with a special focus on vesicle formation and fusion components in the post-Golgi trafficking pathway.

Dynamic behavior of clathrin in Arabidopsis thaliana unveiled by live imaging

Clathrin-coated vesicles (CCV) are necessary for selective transport events, including receptor-mediated endocytosis on the plasma membrane and cargo molecule sorting in the trans-Golgi network (TGN). Components involved in CCV formation include clathrin heavy and light chains and several adaptor proteins that are conserved among plants. Clathrin-dependent endocytosis has been shown to play an integral part in plant endocytosis. However, little information is known about clathrin dynamics in living plant cells. In this study, we have visualized clathrin in Arabidopsis thaliana by tagging clathrin light chain with green fluorescent protein (CLC-GFP). Quantitative evaluations of colocalization demonstrate that the majority of CLC-GFP is localized to the TGN, and a minor population is associated with multivesicular endosomes and the Golgi trans-cisternae. Live imaging further demonstrated the presence of highly dynamic clathrin-positive tubules and vesicles, which appeared to mediate interactions between the TGNs. CLC-GFP is also targeted to cell plates and the plasma membrane. Although CLC-GFP colocalizes with a dynamin isoform at the plasma membrane, these proteins exhibit distinct distributions at newly forming cell plates. This finding indicates independent functions of CLC (clathrin light chains) and dynamin during the formation of cell plates. We have also found that brefeldin A and wortmannin treatment causes distinctly different alterations in the dynamics and distribution of clathrin-coated domains at the plasma membrane. This could account for the different effects of these drugs on plant endocytosis.

Plant clathrin heavy chain: sequence analysis and restricted localisation in growing pollen tubes

Clathrin-coated vesicles were isolated from soybean (Glycine max L.) cells in suspension culture and their purity was assessed using SDS-PAGE, peptide sequencing and electron microscopy. Antibodies raised to these coated vesicles were used to immunoscreen a soybean cDNA library in lambda gt11 and isolate a partial clone of the clathrin heavy chain (HC) gene. Full-length cDNA for soybean clathrin HC was deduced by 5' and 3' cDNA amplification. The cDNA encodes an amino acid sequence of 1,700 residues, which is slightly larger than rat clathrin HC and may account for the reduced mobility of plant clathrin on SDS-PAGE. Insertion of these extra residues is largely confined to the amino and carboxy termini. Other domains within the heavy chain arms, including those implicated in light chain binding and trimerisation, are relatively well conserved between eukaryotes. A computer algorithm to determine alpha-helical coiled-coil structures reveals that only one domain, aligning to residues 1,460-1,489 in rat clathrin HC, has a high probability for coiled-coil structure in all five eukaryotic clathrin HC sequences. This provides further evidence that the interaction between clathrin heavy and light chains is mediated by three bundles of coiled-coils near to the carboxy terminus. In analysing the role of plant clathrin in endocytotic trafficking, as against trafficking from the Golgi apparatus to the vacuole, our attention was focused on membrane recycling in tip-growing pollen tubes. These rapidly growing cells are highly secretory and require a high level of plasma membrane recycling to maintain the tube tip architecture. Monoclonal antibodies to plant clathrin HC confirmed that coated vesicles are relatively abundant in tip-growing pollen tubes of Lilium longiflorum. This analysis also demonstrated that a high proportion of the clathrin present is in an assembled state, suggesting a highly dynamic trafficking pathway. Immunofluorescence analysis of pollen tubes revealed that clathrin localises to the plasma membrane at the apex of the pollen tube tip, which is consistent with high levels of clathrin-mediated membrane recycling. The use of these reagents in conjunction with tip-growing pollen tubes has created a unique opportunity to examine the basis for constitutive endocytosis, so that the more complex question of receptor-mediated pathways in plants can also be assessed.

Analysis of a Nicotiana plumbaginifolia cDNA encoding a novel small GTP-binding protein

Small GTP-binding proteins belonging to the Ras superfamily have been found in evolutionarily divergent organisms. Here, we report the isolation and analysis of a cDNA encoding a putative small GTP-binding protein, designated Rhn1, from the plant, Nicotiana plumbaginifolia. The 21.8-kDa protein has 60% amino acid similarity with the mammalian Rab5 proteins. The Rhn1 protein is encoded by a small multigene family. Northern analysis shows the highest steady-state mRNA levels to be in roots and flowers. Furthermore, the Rhn1 protein has 80% amino acid similarity with an Arabidopsis small GTP-binding protein, designated Rha1.

Purification and characterization of clathrin-coated vesicles from Chlamydomonas

Clathrin-coated vesicles, identified by negative staining with uranyl acetate, were purified from Chlamydomonas reinhardtii. Isolated coated vesicles had diameters ranging from 70 to 140 nm (mean diameter +/- SD of 95 +/- 17 nm, n = 300). These vesicles were markedly heterogeneous in both density and surface charge, as indicated by equilibrium density sedimentation and elution from anion-exchange columns. Highly-purified coated-vesicle fractions contained 2 major polypeptides, identified as the clathrin heavy chain (185 kDa) and the clathrin light chain (40 kDa). Chlamydomonas clathrin heavy chain cross-reacts weakly with an antibody against bovine brain clathrin heavy chain. Coat stability in several buffers was compared to that of bovine brain coated vesicles. Stability was similar, except for a greater stability of Chlamydomonas coated vesicles in 0.5 M Tris at pH 7.0.

Living with clathrin: its role in intracellular membrane traffic

Clathrin polymerization at the cytoplasmic side of the plasma membrane forms coated pits and vesicles that mediate uptake of cell surface receptors. Clathrin-coated vesicles have also been implicated in protein export but definition of their precise role has been controversial. Recent advances in characterization of the clathrin subunits and additional coated vesicle components have identified molecular interactions involved in clathrin polymerization and coated vesicle formation, and have provided new approaches to investigating its function. These studies suggest that clathrin's role, in both inward and outward membrane traffic, is to facilitate receptor transport by a concentration and sorting process that initiates targeting to specific intracellular compartments.

Antibodies to brain clathrin recognise plant coated vesicles

Coated vesicles isolated from carrot suspension culture cells were immune-blotted against four antibodies to porcine brain clathrin. Positive cross-reaction was obtained with three antibodies. Two of these cross-reacted with both the heavy clathrin chain and the putative light chains. Three out of five antibodies immunofluorescently stained permeabilised carrot suspension culture cells.

COATED VESICLES IN THE CYTOPLASM OF THE HOST CELLS IN OPHRYS LUTEA CAV. MYCORRHIZAS (ORCHIDACEAE)

The host cells from endomycorrhizas of Ophrys lutea Cav. contain many coated vesicles associated with dictyosomes, with a membrane system (probably partially coated reticulum) and with the host's sequestration plasmalemma. The contents of coated vesicles stain with neither the PATAg test for polysaccharides nor with ruthenium red. The involvement of coated vesicles in the host/endophyte interactions during the establishment of O. lutea endomycorrhizas is discussed.

Cell-fractionation analysis of glucan synthase I and II distribution and polysaccharide secretion in soybean protoplasts : Evidence for the involvement of coated vesicles in wall biogenesis

The organelles of soybean (Glycine max (L.) Merr.) protoplasts were separated using a recently developed procedure which allows rapid (3-h) recovery of a fraction enriched for coated vesicles (CVs). As determined by marker-enzyme enrichment and ultrastructural analysis of isolated membrane fractions, endoplasmic reticulum, Golgi membranes, glucan-synthase-II (EC 2.4.1.34)-containing membranes (putative plasma membrane), mitochondria, and CVs were enriched in separate fractions in a sucrose density gradient. Glucan synthase I (EC 2.4.1.12) had the highest specific activity in the Golgi-enriched and CV-enriched fractions and was found to comigrate with CVs upon rate-zonal centrifugation of a CV-enriched fraction. For further elucidation of the role of these latter organelles in cell-wall regeneration, freshly isolated protoplasts were pulsed with [(3)H]glucose for 20 min, and the disappearance of label from the organelles was followed for the ensuing 1 h. Although a CV-enriched fraction contained glucan synthase I, it contained very small amounts of labelled polysaccharide during the period of study. Pulse-chase experiments with [(3)H]glucose helped to confirm the role of the Golgi apparatus in secretion of matrix polysaccharides by protoplasts.